in cstrike folder there is a file called cso2012_demo.dem thats the demo file every map the file resets to the current map if you want to save a demo save it somewhere so the demo wont get reseted.
to watch a demo go to console write viewdemo (demo name).
the one you saved make sure to put it back into cstrike folder for example I changed the demo name to test.dem write in console viewdemo test.dem.